1. Field of the Invention
The present invention relates to a non-aqueous electrolyte lithium secondary battery, particularly, to a positive electrode active material for the same.
2. Description of the Prior Art
Non-aqueous electrolyte secondary batteries comprising a lithium or a lithium compound negative electrode are highly promising as the power source for driving cordless electronic as well as electric appliances because they generate a high voltage, providing a high energy density, and therefore, there have been various vigorous studies in the literature.
As the positive electrode active material for the non-aqueous secondary batteries, there have hitherto been proposed transition metal oxides and chalcogen compounds such as LiCoO.sub.2, LiMn.sub.2 O.sub.4, LiFeO.sub.2, LiNiO.sub.2, V.sub.2 O.sub.5, Cr.sub.2 O.sub.5, TiS.sub.2, MoS.sub.2 and the like. These compounds have a layered structure or a tunnel structure of a crystal capable of absorbing or desorbing (intercalating or deintercalating) lithium ions in a reversible manner. In particular, LiCoO.sub.2 and LiNiO.sub.2 are attracting attention in this field of art as the positive electrode active material for a 4 V class non-aqueous electrolyte lithium secondary battery.
Of these compounds, realization of the practical use of LiCoO.sub.2 is under progress because it has a regular and layered structure and can demonstrate a high discharge voltage and a high capacity. Cobalt is, however, a costly element requiring a high manufacturing cost. Moreover, there is a concern about a possible insufficient supply, and a sudden rise in price due to the sudden change in the tense world situation, particularly in the global market. Under these circumstances, LiNiO.sub.2, which is relatively low costly element and can generate a higher capacity exceeding LiCoO.sub.2, is now attracting attention in this field of art, and thus researches and developments are briskly conducted on LiNiO.sub.2 for realizing its practical use. LiNiO.sub.2 has a composition and a structure similar to those of LiCoO.sub.2 and thus a promising material as the positive electrode active material for the lithium secondary batteries to provide a high discharge voltage and a high capacity. With the researches and developments up to the present, the method for synthesizing the same has been improved, and thus a material of superior quality which can demonstrate a high capacity is now available with ease.
As described previously, although the LiNiO.sub.2 has been improved to have a higher initial capacity than that of LiCoO.sub.2, there remains much to be solved in other characteristics. When charging and discharging is performed at a deep depth in order to obtain a large capacity in particular, a serious decrease is created in the lattice constant of the LiNiO.sub.2 and a heavy contraction of the volume of the crystal lattice occurs accordingly. As a result, cracks or cleavages may develop in the particles of the active material and a default is sometimes created in its current-collecting performance, thereby to cause a decrease in the capacity. Further, since such great change occurs in the crystal lattice, a disorder is created in the crystal structure, thereby to trigger a serious deterioration in the capacity of the active material itself. That is, it is believed that when lithium ions are extracted from the crystal in a large quantity during the charging process, the crystal lattice becomes unstable to trigger a rearrangement of the ions in the crystal lattice and the layered structure is disordered, thereby to prevent the lithium ions from diffusing desirably in the crystal and deteriorate the characteristics.
As described previously, LiNiO.sub.2 as the positive electrode active material for the lithium secondary batteries is characterized by its highly excellent initial discharge capacity but lacks a favorable cycle characteristic. The deterioration in the cycle characteristic can be suppressed to a reasonably small degree if only charging and discharging is repeated at a small capacity by selecting a shallow charging depth. If the LiNiO.sub.2 is applied as the positive electrode active material having a higher capacity than LiCoO.sub.2, there is a need to control its crystal structure and suppress the contraction in the crystal lattice and the disorder in the ion arrangement which may occur by a deep depth charging.
A method for controlling the crystal structure includes a substitution of a part of the nickel in the compound with another element. A report on the study appearing in Solid State Ionics, 53-56 (1992), pp. 370-375 describes a result of an attempt to improve the cycle characteristic of LiNiO.sub.2 by substituting a part of the nickel in the compound with another element such as Co.
However, the characteristics of LiNiO.sub.2 as the positive electrode active material for the lithium secondary batteries have not been improved enough, and the charge/discharge characteristic at a high rate and the cycle characteristic at high temperature remain unsatisfactory, in addition to the above-mentioned cycle characteristic at room temperature. Further, in the case of substituting a part of Ni with Co or the like, the cycle characteristic at room temperature is improved but the cycle characteristic at high temperature remains not satisfactory. If the battery is used as the power source for cordless appliances, rapid charging or large current discharging is sometimes required, and thus poor charge/discharge characteristics and the decrease in the discharge capacity by charging and discharging at a high rate become disadvantageous from the practical point of view.
Moreover, since the temperature inside the lithium secondary battery is elevated by large current discharging, an inconvenient restriction may be imposed on the conditions for using the battery if the high temperature characteristics are poor. For instance, the inconvenience is such that the battery cannot be charged immediately after a high rate discharging process. For this, it is important to effectively suppress the deterioration in the characteristics during the operation at high temperature.